Programming Ruby

The Pragmatic Programmer's Guide

module Kernel

The Kernel module is included by class Object, so its methods are available in every Ruby object. The Kernel instance methods are documented in class Object. This section documents the module methods. These methods are called without a receiver and thus can be called in functional form.

module methods

Array

Array( arg ) → anArray

Returns arg.to_a.

Array(1..5) → [1, 2, 3, 4, 5]

Float

Float( arg ) → aFloat

Returns arg converted to a float. Numeric types are converted directly, nil is converted to 0.0, and the rest are converted using arg.to_f.

Float(1) → 1.0
Float(nil) → 0.0
Float("123.456") → 123.456

Integer

Integer( arg ) → anInteger

Converts arg to a Fixnum or Bignum. Numeric types are converted directly (with floating point numbers being truncated). If arg is a String, leading radix indicators (0, 0b, and 0x) are honored. This behavior is different from that of String#to_i.

Converts block to a Proc object (and therefore binds it at the point of call) and registers it for execution when the program exits. If multiple handlers are registered, they are executed in reverse order of registration.

Registers aFile to be loaded (using Kernel::require) the first time that aModule (which may be a String or a symbol) is accessed.

autoload :MyModule, "/usr/local/lib/modules/my_module.rb"

binding

binding → aBinding

Returns a Binding object, describing the variable and method bindings at the point of call. This object can be used when calling eval to execute the evaluated command in this environment. Also see the description of Binding.

Generates a Continuation object, which it passes to the associated block. Performing a cont.call will cause the callcc to return (as will falling through the end of the block). The value returned by the callcc is the value of the block, or the value passed to cont.call. See Continuation for more details. Also see Kernel::throw for an alternative mechanism for unwinding a call stack.

caller

caller( [anInteger] ) → anArray

Returns the current execution stack—an array containing strings in the form “file:line” or “file:line: in `method'”. The optional anInteger parameter determines the number of initial stack entries to omit from the result.

catch executes its block. If a throw is executed, Ruby searches up its stack for a catch block with a tag corresponding to the throw's symbol. If found, that block is terminated, and catch returns the value given to throw. If throw is not called, the block terminates normally, and the value of catch is the value of the last expression evaluated. catch expressions may be nested, and the throw call need not be in lexical scope.

Evaluates the Ruby expression(s) in aString. If aBinding is given, the evaluation is performed in its context. The binding may be a Binding object or a Proc object. If the optional file and line parameters are present, they will be used when reporting syntax errors.

Replaces the current process by running the given external command. If exec is given a single argument, that argument is taken as a line that is subject to shell expansion before being executed. If multiple arguments are given, the second and subsequent arguments are passed as parameters to command with no shell expansion. If the first argument is a two-element array, the first element is the command to be executed, and the second argument is used as the argv[0] value, which may show up in process listings. In MSDOS environments, the command is executed in a subshell; otherwise, one of the exec(2) system calls is used, so the running command may inherit some of the environment of the original program (including open file descriptors).

exec "echo *" # echoes list of files in current directory
# never get here
exec "echo", "*" # echoes an asterisk
# never get here

exit

exit( anInteger=0 )

Initiates the termination of the Ruby script by raising the SystemExit exception. This exception may be caught. The optional parameter is used to return a status code to the invoking environment.

Creates a subshell. If a block is specified, that block is run in the subshell, and the subshell terminates with a status of zero. Otherwise, the fork call returns twice, once in the parent, returning the process id of the child, and once in the child, returning nil. The child process can exit using Kernel::exit! to avoid running any at_exit functions. The parent process should use Process::wait to collect the termination statuses of its children; otherwise, the operating system may accumulate zombie processes.

Returns (and assigns to $_) the next line from the list of files in ARGV (or $*), or from standard input if no files are present on the command line. Returns nil at end of file. The optional argument specifies the record separator. The separator is included with the contents of each record. A separator of nil reads the entire contents, and a zero-length separator reads the input one paragraph at a time, where paragraphs are divided by two consecutive newlines. If multiple filenames are present in ARGV, gets(nil) will read the contents one file at a time.

ARGV << "testfile"
print while gets

produces:

This is line one
This is line two
This is line three
And so on...

global_variables

global_variables → anArray

Returns an array of the names of global variables.

global_variables.grep /std/ → ["$stdin", "$stderr", "$stdout"]

gsub

gsub( pattern, replacement ) → aString

gsub( pattern ) {| | block } → aString

Equivalent to $_.gsub..., except that $_ receives the modified result.

Loads and executes the Ruby program in the file aFileName. If the filename does not resolve to an absolute path, the file is searched for in the library directories listed in $:. If the optional wrap parameter is true, the loaded script will be executed under an anonymous module, protecting the calling program's global namespace. Any local variables in the loaded file will not be propagated to the loading environment.

local_variables

local_variables → anArray

Returns the names of the current local variables.

fred = 1
for i in 1..10
# ...
end
local_variables → ["fred", "i"]

loop

loop {| | block }

Repeatedly executes the block.

loop {
print "Input: "
break if !gets or $_ =~ /^qQ/
# ...
}

open

open( aString [, aMode [perm]] ) → anIO or nil

open( aString [, aMode [perm]] ) {| anIO | block } → nil

Creates an IO object connected to the given stream, file, or subprocess.

If aString does not start with a pipe character (“|”), treat it as the name of a file to open using the specified mode defaulting to “r” (see the table of valid modes). If a file is being created, its initial permissions may be set using the integer third parameter.

If a block is specified, it will be invoked with the File object as a parameter, and the file will be automatically closed when the block terminates. The call always returns nil in this case.

If aString starts with a pipe character, a subprocess is created, connected to the caller by a pair of pipes. The returned IO object may be used to write to the standard input and read from the standard output of this subprocess. If the command following the “|” is a single minus sign, Ruby forks, and this subprocess is connected to the parent. In the subprocess, the open call returns nil. If the command is not “-”, the subprocess runs the command. If a block is associated with an open("|-") call, that block will be run twice—once in the parent and once in the child. The block parameter will be an IO object in the parent and nil in the child. The parent's IO object will be connected to the child's $stdin and $stdout. The subprocess will be terminated at the end of the block.

For each object, directly writes anObject.inspect followed by the current output record separator to the program's standard output. p bypasses the Ruby I/O libraries.

p self

produces:

main

print

print( [anObject]* ) → nil

Prints each object in turn to $defout. If the output field separator ($,) is not nil, its contents will appear between each field. If the output record separator ($\) is not nil, it will be appended to the output. If no arguments are given, prints $_. Objects that aren't strings will be converted by calling their to_s method.

Creates a new procedure object from the given block. Equivalent to Proc.new.

aProc = proc { "hello" }
aProc.call → "hello"

putc

putc( anInteger ) → anInteger

Equivalent to $defout.putc(anInteger).

puts

puts( [args]* ) → nil

Equivalent to $defout.puts(args).

raise

raise

raise( aString )

raise( anException [, aString [anArray]] )

With no arguments, raises the exception in $! or raises a RuntimeError if $! is nil. With a single String argument, raises a RuntimeError with the string as a message. Otherwise, the first parameter should be the name of an Exception class (or an object that returns an Exception when sent exception). The optional second parameter sets the message associated with the exception, and the third parameter is an array of callback information. Exceptions are caught by the rescue clause of begin...end blocks.

Converts max to an integer using max1 = max.to_i.abs. If the result is zero, returns a pseudorandom floating point number greater than or equal to 0.0 and less than 1.0. Otherwise, returns a pseudorandom integer greater than or equal to zero and less than max1. Kernel::srand may be used to ensure repeatable sequences of random numbers between different runs of the program.

Equivalent to Kernel::gets, except readline raises
EOFError at end of file.

readlines

readlines( [aString=$/] ) → anArray

Returns an array containing the lines returned by
calling Kernel.gets(aString) until the end of file.

require

require( aString ) → true or false

Ruby tries to load the library named aString, returning true if successful. If the filename does not resolve to an absolute path, it will be searched for in the directories listed in $:. If the file has the extension “.rb”, it is loaded as a source file; if the extension is “.so”, “.o”, or “.dll”, (Or whatever the default shared library extension is on the current platform.) Ruby loads the shared library as a Ruby extension. Otherwise, Ruby tries adding “.rb”, “.so”, and so on to the name. The name of the loaded feature is added to the array in $". A feature will not be loaded if it already appears in $". require returns true if the feature was successfully loaded.

Performs a low-level select call, which waits for data to become available from input/output devices. The first three parameters are arrays of IO objects or nil. The last is a timeout in seconds, which should be an Integer or a Float. The call waits for data to become available for any of the IO objects in readArray, for buffers to have cleared sufficiently to enable writing to any of the devices in writeArray, or for an error to occur on the devices in errorArray. If one or more of these conditions are met, the call returns a three-element array containing arrays of the IO objects that were ready. Otherwise, if there is no change in status for timeout seconds, the call returns nil. If all parameters are nil, the current thread sleeps forever.

select( [$stdin], nil, nil, 1.5 ) → [[#<IO:0x401ba090>], [], []]

set_trace_func

set_trace_func( aProc ) → aProc

set_trace_func( nil ) → nil

Establishes aProc as the handler for tracing, or disables tracing if the parameter is nil. aProc takes up to six parameters: an event name, a filename, a line number, an object id, a binding, and the name of a class. aProc is invoked whenever an event occurs. Events are: c-call (call a C-language routine), c-return (return from a C-language routine), call (call a Ruby method), class (start a class or module definition), end (finish a class or module definition), line (execute code on a new line), raise (raise an exception), and return (return from a Ruby method). Tracing is disabled within the context of aProc.

Invoked with a symbol id whenever a singleton method is added to a module or a class. The default implementation in Kernel ignores this, but subclasses may override the method to provide specialized functionality.

class Test
def Test.singleton_method_added(id)
puts "Added #{id.id2name} to Test"
end
def a() end
def Test.b() end
end
def Test.c() end

produces:

Added singleton_method_added to Test
Added b to Test
Added c to Test

sleep

sleep( [aNumeric] ) → aFixnum

Suspends the current thread for aNumber seconds (which may be a Float with fractional seconds). Returns the actual number of seconds slept (rounded), which may be less than that asked for if the thread was interrupted by a SIGALRM, or if another thread calls Thread#run. An argument of zero causes sleep to sleep forever.

Returns the string resulting from applying aFormatString to any additional arguments. Within the format string, any characters other than format sequences are copied to the result. A format sequence consists of a percent sign, followed by optional flags, width, and precision indicators, then terminated with a field type character. The field type controls how the corresponding sprintf argument is to be interpreted, while the flags modify that interpretation. The flag characters are shown in Table 23.1, and the field type characters are listed in Table 23.2.

The field width is an optional integer, followed optionally by a period and a precision. The width specifies the minimum number of characters that will be written to the result for this field. For numeric fields, the precision controls the number of decimal places displayed. For string fields, the precision determines the maximum number of characters to be copied from the string. (Thus, the format sequence %10.10s will always contribute exactly ten characters to the result.)

Table 23.1 : sprintf flag characters

Flag

Applies to

Meaning

‿ (space)

bdeEfgGioxXu

Leave a space at the start of positive numbers.

#

beEfgGoxX

Use an alternative format. For the conversions `o', `x', `X', and `b', prefix the result with
“0”, “0x”, “0X”, and “0b”, respectively. For `e',
`E', `f', `g', and 'G', force a decimal point to be added,
even if no digits follow. For `g' and 'G', do not remove
trailing zeros.

+

bdeEfgGioxXu

Add a leading plus sign to positive numbers.

-

all

Left-justify the result of this conversion.

0 (zero)

all

Pad with zeros, not spaces.

*

all

Use the next argument as the field width. If negative, left-justify the result. If the asterisk is
followed by a number and a dollar sign, use
the indicated argument as the width.

Table 23.2 : sprintf field types

Field

Conversion

b

Convert argument as a binary number.

c

Argument is the numeric code for a single character.

d

Convert argument as a decimal number.

E

Equivalent to `e', but uses an uppercase E to indicate the exponent.

e

Convert floating point argument into exponential notation with one digit before the decimal point. The precision
determines the number of fractional digits (defaulting to six).

f

Convert floating point argument as [‿-]ddd.ddd, where the precision determines the number of digits after
the decimal point.

G

Equivalent to `g', but use an uppercase `E' in exponent form.

g

Convert a floating point number using exponential form if the exponent is less than -4 or greater than or
equal to the precision, or in d.dddd form otherwise.

i

Identical to `d'.

o

Convert argument as an octal number.

s

Argument is a string to be substituted. If the format sequence contains a precision, at most that many characters
will be copied.

Seeds the pseudorandom number generator to the value of aNumber.to_i.abs. If aNumber is omitted or zero, seeds the generator using a combination of the time, the process id, and a sequence number. (This is also the behavior if Kernel::rand is called without previously calling srand, but without the sequence.) By setting the seed to a known value, scripts can be made deterministic during testing. The previous seed value is returned. Also see Kernel::rand.

sub

sub( pattern, replacement ) → $_

sub( pattern ) { block } → $_

Equivalent to $_.sub(args), except that $_
will be updated if substitution occurs.

sub!

sub!( pattern, replacement ) → $_ or nil

sub!( pattern ) { block } → $_ or nil

Equivalent to $_.sub!(args).

syscall

syscall( aFixnum[, args]* ) → anInteger

Calls the operating system function identified by aFixnum,
passing in the arguments, which must be either String objects, or
Integer objects that ultimately fit within a native long.
Up to nine parameters may be passed (14 on the
Atari-ST). The function identified
by Fixnum is system dependent. On some Unix systems, the
numbers may be obtained from a header file called
syscall.h.

syscall 4, 1, "hello\n", 6 # '4' is write(2) on our box

produces:

hello

system

system( aCmd[, args]* ) → true or false

Executes aCmd in a subshell, returning true if the command was found and ran successfully, false otherwise. A detailed error code is available in $?. The arguments are processed in the same way as for Kernel::exec.

system("echo *")
system("echo", "*")

produces:

config.h main.rb
*

test

test(aCmd, file1[, file2] ) → anObject

Uses the integer aCmd to perform various tests on file1 (Table 23.3) or on file1 and file2 (Table 23.4).

Table 23.3 : File tests with a single argument

Integer

Description

Returns

?A

Last access time for file1

Time

?b

True if file1 is a block device

true or false

?c

True if file1 is a character device

true or false

?C

Last change time for file1

Time

?d

True if file1 exists and is a directory

true or false

?e

True if file1 exists

true or false

?f

True if file1 exists and is a regular file

true or false

?g

True if file1 has the setgid bit set (false under NT)

true or false

?G

True if file1 exists and has a group ownership equal to the caller's group

true or false

?k

True if file1 exists and has the sticky bit set

true or false

?l

True if file1 exists and is a symbolic link

true or false

?M

Last modification time for file1

Time

?o

True if file1 exists and is owned by the caller's effective uid

true or false

?O

True if file1 exists and is owned by the caller's real uid

true or false

?p

True if file1 exists and is a fifo

true or false

?r

True if file is readable by the effective uid/gid of the caller

true or false

?R

True if file is readable by the real uid/gid of the caller

true or false

?s

If file1 has nonzero size, return the size, otherwise return nil

Integer or nil

?S

True if file1 exists and is a socket

true or false

?u

True if file1 has the setuid bit set

true or false

?w

True if file1 exists and is writable by the effective uid/gid

true or false

?W

True if file1 exists and is writable by the real uid/gid

true or false

?x

True if file1 exists and is executable by the effective uid/gid

true or false

?X

True if file1 exists and is executable by the real uid/gid

true or false

?z

True if file1 exists and has a zero length

true or false

Table 23.4 : File tests with two arguments

Integer

Description

?-

True if file1 is a hard link to file2

?=

True if the modification times of file1 and file2 are equal

?<

True if the modification time of file1 is prior to that of file2

?>

True if the modification time of file1 is after that of file2

throw

throw( aSymbol [, anObject] )

Transfers control to the end of the active catch block waiting for aSymbol. Raises NameError if there is no catch block for the symbol. The optional second parameter supplies a return value for the catch block, which otherwise defaults to nil. For examples, see Kernel::catch.

trace_var

trace_var( aSymbol, aCmd ) → nil

trace_var( aSymbol ) {| val | block } → nil

Controls tracing of assignments to global variables. The parameter aSymbol identifies the variable (as either a string name or a symbol identifier). cmd (which may be a string or a Proc object) or block is executed whenever the variable is assigned. The block or Proc object receives the variable's new value as a parameter. Also see Kernel::untrace_var.

Specifies the handling of signals. The first parameter is a signal name (a string such as “SIGALRM”, “SIGUSR1”, and so on) or a signal number. The characters “SIG” may be omitted from the signal name. The command or block specifies code to be run when the signal is raised. If the command is the string “IGNORE” or “SIG_IGN”, the signal will be ignored. If the command is “DEFAULT” or “SIG_DFL”, the operating system's default handler will be invoked. If the command is “EXIT”, the script will be terminated by the signal. Otherwise, the given command or block will be run.

The special signal name “EXIT” or signal number zero will be invoked just prior to program termination.

Removes tracing for the specified command on the given global variable and returns nil. If no command is specified, removes all tracing for that variable and returns an array containing the commands actually removed.